1. Field of the Invention
The present invention relates to a noise controller which cancels noise by outputting from a speaker a noise cancelling sound having a phase opposite to and a sound pressure equal to those of noise produced by an engine, a motor or the like. More specifically, the invention relates to judging any deviation from initial equalization-forming conditions that compensate for the attenuation of frequency bands and the transfer characteristics caused by the delay of propagation time in the transmission path of the noise controller.
2. Description of the Prior Art
Passive silencing devices such as mufflers and the like have heretofore been used to reduce the noise generated from internal combustion engines and the like needing, however, improvements from the standpoint of size and silencing characteristics. There has, on the other hand, been proposed an active noise controller which cancels the noise by outputting from a speaker a noise cancelling sound having a phase opposite to and a sound pressure equal to those of the noise generated from the source of sound. However, the active noise controller was not readily put into practical use because it lacked certain frequency characteristics and stability. In recent years, however, there have been proposed many practical noise controllers along with developments in the technology for processing digital signals and in the art for handling wide ranges of frequencies (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 63-311396).
A digital signal processor (DSP) in the conventional noise controller uses an adaptive filter of the FIR (finite impulse response) type which forms a signal for cancelling noise upon receiving a reference signal which is a signal to be controlled, detects a residual sound which is the result of cancellation, and performs a feedback control using the residual sound as an error signal such that the level of the residual sound is minimized. In this feedback control, furthermore, the level of the error signal can be minimized by controlling the filter coefficient of the adaptive filter. The reference signal applied to the adaptive filter can be obtained by synthesizing the noise cancelling signal formed by itself and the error signal that is detected.
Here, the noise controller uses a speaker for producing a noise cancelling sound and a microphone for detecting an error signal, and space through which sound waves propagate exists between the speaker and the microphone. Therefore, frequency bands are attenuated and the propagation time is delayed for the relevant transmission band. Compensating for the transmission characteristics in the transmission band is generally called initial equalization. The processing of initial equalization is carried out to form a filter coefficient of the adaptive filter.
However, there remains a first problem in that if the speaker, microphone and the like constituting the noise controller become defective or deteriorate, the accuracy of the initial equalization becomes extremely poor, and the effect of noise control is not obtained to a sufficient degree.
In view of the above-mentioned problem, therefore, it is an object of the present invention to provide a noise controller which is capable of judging a decrease in accuracy of the initial equalization at an early stage.
There further remains a second problem in that when the noise controller is used under different conditions from the space in which it was originally installed, the initial equalization deviates from the preset initial equalization, and abnormal operation occurs if the noise controller is used under this condition.
In view of the above-mentioned problem, therefore, the object of the present invention is to provide a noise controller which is capable of judging whether the initial equalization is proper or not in response to a change in the conditions in which the noise controller is used.